Gerald81

Does anyone know if there is an aftermarket or Subaru master cylinder that is larger than the 1 1/16 inch or 26.99mm STI ones?

My car is a 2007 STI which already comes with the 26.99mm ones. After I upgraded the fronts to the STI RA-R 6-pot calipers, the pedal stroke seems to be very slightly softer and longer. I was fine with it for awhile but now I've put on the R205 4-pot calipers on the rears, and the pedal stroke is even softer and longer than before.

Still somewhat driveable, but I was wondering if a slightly larger MC would fix this, given the increase in piston surface area moving from the stock 4/2 setup to 6/4 now?

NOSSY_89

Start with the easy things first. Bleed the brakes and add new/better brake fluid. Have a look into braided lines and a brake stopper.

Gerald81

Thanks, right now I have tried the following:

- Brakes were bled with vacuum
- I'm using Motul Dot 4 but might try something else if that helps
- Already have SS braided lines
- Already have a brake stopper

For now it really seems to me as if the master cylinder is not large enough for the increase from 4/2 to 6/4 pots, but that's just my layman perspective. Admittedly not an expert in this regard.

NOSSY_89

I'm not familiar with that method so unsure of how you're supposed to do it or any issues you might need to consider.

This might sound obvious but bleed the brakes starting from the caliper that is furthest from the brake cylinder.

I'm running a different set up to you. Tarox 6 pots at the front and stock Subaru 1 pots at the back. I used to have a car with 1 pots at the front too and going from the different set up I don't remeber the pedal feeling any different. I don't know if the brake cylinder is the same but I can't imagine them being different.

Based on your description it does seem like the brakes need to be bled.

Just doing a bit of Googleing and it could be your brake cylinder is a bit tired and in need of replacing or fixing.

Gerald81

Thanks, I might have to do that. Waiting on some piston seal kits to arrive so might refresh the calipers and change the fluids and bleed at the same time.

What fluids would you recommend for a nice, stiff pedal feel?

Gerald81

Also interesting with your experience on upgrading from a 1 pot front. I too used to drive a Subaru with 1 pot front and drums on the rear, when I upgraded to WRX 4/2 pot calipers, the pedal stroke became extremely soft and long. Much worse than what I have now. Assuming the work shop bled the brakes properly, I was thinking it was due to the master cyclinder being under-sized for the upgraded brake system.

NOSSY_89

Dot 4 or Dot 5.1 from what I have read. Do not use Dot 5 because that can't be mixed with the others.

I added some additional information to my reply that you might not have seen. I was adding it as you were replying.

Check your brake cylinder it might be a bit tired and in need of replacing or fixing.

Gerald81

Thanks, will look into that as well!

Gerald81

Would something like the below be a bolt-on fit for our cars?

https://www.wilwood.com/MasterCylind...o=260-14960-BK

Don Clark

From what I can see in the parts databases the RHD MY07 Impreza STI runs a 26401AC181 master cylinder
The MY08 on RHD Hatch/Saloon runs a 26401AG051 master cylinder which includes the R205/S206/TsRA and NBR models

DDS789

iTrader: (2)

Also worth considering there may be air in the abs system

Sometimes the abs needs actuating to get it out

Could be a long shot but had it before on a different car

Gerald81

Thanks again for the replies. I know everyone's first reaction to this issue is bleeding, etc. I really doubt this is the issue, and in any case the work itself was done by the shop's boss who's been working on Subarus since the GC days, build competition Subarus and owns a Subaru himself. Also, I have all the usual braking mods such as SS braided hoses, brake stopper, etc.

I did a fair bit of research before pulling the trigger on this upgrade.

Firstly, I noted that the stock GD STI (with 4/2 pots) and the RA-R (with 6/2 pots) both shared the same ancillary braking hardware, i.e. exact same part numbers.

Secondly, I noted that the stock GR STI (with 4/2 pots) and the R205 (with 6/4 pots) both also shared the same ancillary braking hardware.

For the two reasons above, I surmised that Subaru deemed that moving from 4/2 to 6/2 or even 6/4 would not significantly affect pedal feel. However, I personally did feel the difference (however mild) each time I upgraded my brakes, first to 6/2 and now more recently to 6/4.

Doing more research, and also based on my prior experience with a more drastic upgrade from a 1 pot to 4/2 pots setup where the pedal feel/travel really went down the drain, I've come to the conclusion that the MC is somewhat undersized now. I would really like to try a larger one such as a 1-1/8", but only option is aftermarket and I have no idea how to verify if these would be compatible, as I can't find much in terms of specs (e.g. stroke) for the stock MCs apart from the bore size.

I wonder if there are other considerations as well, such as the ABS system which could affect the pedal feel even under normal conditions when the ABS is not activated?

Don Clark

No problems experienced with Alcons (6/4) on a GVF with brake stopper, braided lines and OEM mc

NOSSY_89

A lot of people have gone bigger brakes like Don has and no one has really complained about the Brake Cylinder needing to be changed. It might be worth rebuilding your Brake Cylinder.

If you have a search someone else had a similar issue and one of the comments was to go for a newer unit and how much difference it made.

Something else to consider, time is money, if you're a garage and for whatever reason you need to do the two man method to bleed brakes then you have effectively two mechanics out of action. I'm not saying the garage you used isn't good at all its just something to consider.

It could also be the ABS system like DDS789 suggested.

Try bleeding the brakes yourself using the 2 man method and let us know how you get on. It will be cheaper and brilliant if it fixes your issue.

All the best

1509joe

iTrader: (2)

Type r AP 6 pot fronts AP 4 pots rear if anything pedal would be too hard (everything checked) Obviously no ABS so I would check everything again.
Brakes the very best.

John 37

+1 for master cylinder as the culprit.
I've heard cases of vacuum bleeders sucking air in past tired seals. The seals work ok to hold prsssure in but a vac bleeder is stressing them in the wrong direction.
Try the old way of bleeding by pumping the pedal.